Many applications exist in which fine solids, e.g., pellets, are unloaded and conveyed using a gravity feed system. One such application is found in cable manufacturing in which polymeric resin is used for the manufacture of one or more sheaths that protect the conducting core. These resins are typically delivered to the manufacturing plant in the form of pellets, transferred to a holding and/or feed hopper, and then transferred under the force of gravity from the holding hopper to a conveying pipe. Alternatively, the pellets are delivered in large bags or sacks from which they are discharged into the conveying pipe, typically through an intermediate hopper that funnels the pellets into the pipe. The pellets are then conveyed through the pipe under any suitable force, e.g., auger screw, pneumatic pressure, etc., to an extruder from which molten resin is extruded onto a wire or cable in the form of a sheath.
While agglomeration of fine solids, particularly polymeric fine solids, can occur under many different conditions, agglomeration is particularly troubling during the transfer of fine solids from a relatively large space to a relatively small space. For example, the transfer of fine solids from a storage bin, large feed hopper or sack that can contain hundreds, if not thousands, of pounds of fine material, to a receptacle that usually contains less than a hundred pounds of material at any one time, e.g., an extruder, usually involves the passage of the fine material through a narrowed or tapered section of the hopper that acts as a funnel for directing the flow of the material into a conveying receptacle or vessel, e.g., a pipe, for transfer of the fine material to processing equipment. During the passage of the fine material through this tapered section of the equipment, the individual particles of the material experience increased pressure and a tendency to bind or agglomerate with the individual particles in which they are in contact. Certain resins, e.g., crosslinked polyethylene (XLPE), a common material in the manufacture of insulation coverings for wire and cable, are more prone to such agglomeration than others, particularly under extreme temperatures, e.g., the heat experienced in a non-air-conditioned or limited air-conditioned manufacturing facility located in a tropical or sub-tropical climate during the summer, or the cold experienced in a non-heated or limited heated manufacturing facility located in a northern climate during the winter.
If the agglomerations are small enough, they can pass through the system from storage bin or holding hopper to extruder without disruption of the overall process. Large agglomerations, however, can and often do disrupt the overall process by clogging narrow or pinch points in the system. The downstream equipment that relies on a steady feed of fine solids from the feed vessel may not be timely alerted to the reduction or cessation of flow through the system which in turn can result in unacceptable fluctuations in the dimensions and/or quality of the final product. Moreover, removing the agglomerations from the system can be exceedingly difficult, and it may introduce contaminates into the material-in-process.